Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, quaternary, and quinary alloys of gallium, aluminum, indium, boron, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
As described by Wampler et al. in Hydrogen release from magnesium-doped GaN with clean ordered surfaces, J. Appl. Phys., Vol. 94, No. 9, P. 5682 (2003), “Mg-doped GaN, grown by metal-organic vapor phase epitaxy (MOVPE) has low conductivity due to passivation of acceptors by hydrogen (H) incorporated during growth. The passivation occurs through the formation of an electrically neutral Mg—H complex. The p-type conductivity required for devices is achieved through postgrowth activation of Mg by thermal annealing, which releases H from the material. However, the high anneal temperatures [above 700° C. . . . ], required to activate Mg acceptors, complicates device fabrication. . . . [A]ctivation does occur at lower temperatures when the GaN surface is coated with metal films, or when annealing is done in an oxidizing ambient.”